Ceramics in general have many uses, especially in applications requiring light weight, high temperature resistant materials. However, the use of ceramics is often limited due to their inherent brittleness. As opposed to monolithic ceramics, ceramic-ceramic composites offer greater structural reliability because they generally do not catastrophically fail upon initial macroscopic cracking of the matrix. The increased toughness achieved by ceramic-ceramic composites allows for use in applications where brittle, monolithic ceramics are unsatisfactory.
A particular monolithic ceramic which has been of considerable interest in recent years is silicon nitride. Properties such as retention of mechanical strength and creep resistance at high temperature and exceptionally good resistance to thermal shock, for a brittle material, have led to the use of silicon nitride in applications such as high temperature engine components. Formation of silicon nitride components may be achieved by the direct reaction of silicon powder with nitrogen to form Si.sub.3 N.sub.4 powder, followed by conventional sintering or hot pressing techniques to form a shaped article. Alternatively, silicon nitride components may be produced directly by a method known as reaction bonding. In this process, a silicon powder compact is converted into Si.sub.3 N.sub.4 by heating in a nitrogen atmosphere. The reaction bonding technique has the merit that only slight overall volume change occurs during the nitridation process, so that complex, accurately dimensioned shapes can be produced in a single stage from a shaped billet of silicon powder. However, the use of silicon nitride in components requiring, for example, structural integrity, has been limited due to the inherent brittleness of the material.
Known ceramic-ceramic composites are produced by conventional powder processing techniques which involve mixing dissimilar ceramic powders followed by a heating process to obtain a dispersion of one of the starting ceramics in a matrix of the other starting ceramic. The grain size of the resultant ceramic composite depends upon the size of the starting powders used. Difficulties are encountered as the size of the starting powders is decreased due to a tendency for the very fine powders to agglomerate. Also, very fine ceramic powders are often not commercially available. Further, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature. The temperatures involved in conventional consolidation techniques are typically very high, which may result in growth of the ceramic particles along with unwanted reactions between the different ceramic components. As an alternative, sol-gel technology has been investigated because this route offers the potential for finer particles of more uniform dispersion than possible by conventional processing. See H. Palmour III et al., Processing of Crystalline Ceramics, Plenum Press (1978), hereby incorporated by reference. However, sol-gel techniques incorporate a wet processing step which complicates processing, i.e., the gel must first be dried and/or calcined. This technique also includes the use of relatively high temperature which is likely to promote grain growth and reactions between different components.
Refractories, such as those referred to above, can be hot pressed at very high temperatures using a non-oxidizing or inert atmosphere, such as nitrogen, helium, argon or a vacuum to form ceramics. However, such high temperatures similarly favor undersirable rapid particle growth and undesireable side reactions. See, for example, U.S. Pat. Nos. 2,670,301; 2,839,413; 3,011,983; 3,291,623; and 4,512,946.
For the past several years, extensive research has been devoted to the development of metal-ceramic composites, such as aluminum reinforced with carbon, boron, silicon carbide, silica, or alumina fibers, whiskers, or particles. Metal-ceramic composites with good high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix. However, this metal ceramic composite technology has not heretofore been extended to include ceramic matrices. Prior art techniques for the production of metal-ceramic composites may be broadly categorized as powder metallurgical approaches, molten metal techniques, and internal oxidation processes.
The powder metallurgical type production of such dispersion-strengthened composites would ideally be accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with the oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques or conventional procedures such as ball-milling may be used to mix the powder. Standard powder metallurgy techniques are then used to form the final composite. Conventionally, however, the ceramic component is large, that is, greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials because their production is energy intensive, time consuming, and costly in capital equipment. Furthermore, production of very small particles inevitably leads to contamination of the particles with oxides, nitrides, and materials from various sources. The presence of these contaminants inhibits particulate-to-metal bonding, which in turn compromises the mechanical properties of the resultant composites.
Alternatively, it is known that proprietary processes exist for the direct addition of appropriately coated ceramics to molten metals. Further, molten metal infiltration of a continuous ceramic skeleton has been used to produce composites. In most cases, elaborate particle coating techniques have been developed to protect the ceramic particles from the molten metal during admixture or molten metal infiltration, and to improve bonding between the metal and ceramic. Techniques such as these have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is only suitable for large particulate ceramics (for example, greater than 1 micron) and whiskers, because of the high pressures involved for infiltration. The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the intersticies. Such a technique is illustrated in U.S. Pat. No. 4,444,603, of Yamatsuta et al, issued Apr. 24, 1984 and hereby incorporated by reference. Because of the necessity for coating techniques and molten metal handling equipment capable of generating extremely high pressures, molten metal infiltration has not been a practical process for making metal-ceramic composites.
Internal oxidation of a metal containing a more reactive component has been used to produce dispersion strengthened metals, such as internally oxidized aluminum in copper. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. This technique, although limited to relatively few systems because the two metals utilized must have a wide difference in chemical reactivity, has offered a feasible method for dispersion hardening. However, the highest possible level of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness, and the like. In addition, oxides are typically not wetted by the metal matrix, so that interfacial bonding is not optimum.
Because of the above-noted difficulties with conventional processes, preparation of metal-ceramic composites with submicron ceramic dispersoids for commercial applications has been extremely expensive.
In U.S. Pat. No. 2,852,366 to Jenkins, hereby incorporated by reference, it is taught that up to 10 by weight of a metal complex can be incorporated into a base metal or alloy. The patent teaches blending, pressing, and sintering a mixture of a base metal, a compound of the base metal and a non-metallic complexing element, and an alloy of the base metal and the complexing metal. Thus, for example, the reference teaches mixing powders of nickel, a nickel-boron alloy, and a nickel-titanium alloy, pressing, and sintering the mixed powders to form a coherent body in which a stabilizing unprecipitated "complex" of titanium and boron is dispersed in a nickel matrix. Precipitation of the complex phase is specifically avoided.
In U.S. Pat. No. 3,194,656, hereby incorporated by reference, Vordahl teaches the formation of a ceramic phase, such as TiB.sub.2 crystallites, by melting a mixture of eutectic or near eutectic alloys. It is essential to the process of Vordahl that at least one starting ingredient has a melting point substantially lower than that of the matrix metal of the desired final alloy. There is no disclosure of the initiation of an exothermic second phase-forming reaction at or near the melting point of the matrix metal.
Bredzs et al, in U.S. Pat. Nos. 3,145,697; 3,547,673; 3,666,436; 3,672,849; 3,690,849; 3,690,875; and 3,705,791, hereby incorporated by reference, teach the preparation of cermet coatings, coated substrates, and alloy ingots, wherein an exothermic reaction mechanism forms an in-situ precipitate dispersed in a metal matrix. Bredzs et al rely on the use of alloys having a depressed melting temperature, preferably eutectic alloys, and thus do not initiate a second phase-forming exothermic reaction at or near the melting temperature of the matrix metal.
DeAngelis, in U.S. Pat. No. 4,514,268, hereby incorporated by reference, teaches reaction sintered cermets having very fine grain size. The method taught involves the dual effect of reaction between and sintering together of admixed particulate reactants that are shaped and heated at temperatures causing an exothermic reaction to occur and be substantially completed. The reaction products are at least partially sintered together by holding the reaction mass at the high temperatures attained to form a body comprising aluminum metal and a ceramic skeleton. Thus, this reference relates to a product with sintered ceramic bonds suitable for use in contact with molten metal.
Backerud, in U.S. Pat. No. 3,785,807, hereby incorporated by reference, teaches the concept of preparing a master alloy for aluminum, containing titanium diboride. The patentee dissolves and reacts titanium and boron in molten aluminum at a high temperature, but requires that titanium aluminide be crystallized at a lower temperature around the titanium diboride formed. Thus, the patent teaches formation of a complex disperoid.
Recently, there has been considerable effort to produce various ceramic materials by gasless combustion synthesis. See, for example, W. L. Frankhouser et al., Synthesis of Refractory Compounds with Gasless Combustion Reactions, Final Report No. SPC 931, DARPA. (Sept., 1983), hereby incorporated by reference. This method typically involves the direct combination of elemental materials to produce ceramics. The non-isothermal reaction is marked by a substantial energy release which heats the materials above the melting point of the product ceramic.
In recent years, numerous ceramics have been formed using a process termed "self-propagating high-temperature synthesis" SHS). It involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. The SHS process involves mixing and compacting powders of the constituent elements and igniting a portion of a green compact with a suitable heat source. The source can be electrical impulse, laser, thermite, spark, etc. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation at high temperatures, rather than bulk heating over long periods at lower temperatures. Exemplary of these techniques are the patents of Merzhanov et al, U.S. Pat. Nos. 3,726,643; 4,161,512; and 4,431,448, among others, hereby incorporated by reference.
In U.S. Pat. No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compounds by mixing at least one metal selected from Groups IV, V, and VI of the Periodic System with a non-metal, such as carbon, boron, silicon, sulfur, or liquid nitrogen, and heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, a process is taught for preparing titanium carbide by ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, absent a binder.
When the SHS process is used with an inert metal phase, it is generally performed with a relatively high volume fraction of ceramic and a relatively low volume fraction of metal (typically 10 percent and below, and almost invariably below 30 percent). The product is a dense, sintered material wherein the relatively ductile metal phase acts as a binder or consolidation aid which, due to applied pressure, fills voids, etc., thereby. increasing density. The SHS process with inert metal phase occurs at higher temperatures than the in-situ precipitation process used in conjunction with the present invention, and is non-isothermal, yielding sintered ceramic particles having substantial variation in size.
U.S. Pat. No. 4,431,448 teaches preparation of a hard alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal or alloy, such as an alloy of copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the ignition, resulting in an alloy comprising titanium diboride, titanium carbide, and up to about 30 percent binder metal. This reference, however, is limited to the use of Group I-B metals or alloys, such as copper and silver, as binders. Products made by this method have low density, and are subjected to subsequent compression and compaction to achieve a porosity below 1 percent.
U.S. Pat. No. 4,540,546 to Giessen et al, hereby incorporated by reference, teaches a method for rapid solidification processing of a multiphase alloy. In this process two starting alloys react in a mixing nozzle in which a "Melt Mix Reaction" takes place between chemically reactable components in the starting alloys to form submicron particles of the resultant compound in the final alloy. The mixing and chemical reaction are performed at a temperature which is at or above the highest liquidus temperature of the starting alloys, but which is also substantially below the liquidus temperature of the final alloy, and as close to the solidus temperature of the final alloy as possible. The mixing and reaction also occurs in a non-reactive atmosphere (H, He, Ar, N) or a cooling fluid. While dispersion-strengthened alloys can be produced by this technique, there appear to be a number of inherent difficulties. First, processing is technically complex, requiring multiple furnaces. Second, efficient mixing is important if fine dispersions are to be consistently produced. Lastly, very high degrees of superheat will be required to completely dissolve the rapid solidification alloying elements in order to produce high loading of dispersoid, which necessarily accentuates particle growth, for example, in composites containing 10-20% dispersoid.
European patent application No. 113,249, filed Dec. 29, 1983, by Reeve et al., teaches a cermet material comprising an intergrown network of a minor proportion of ceramic such as TiB.sub.2 in a metal matrix such as Al. The cermet is prepared by forming a minor proportion by weight of a non-particulate ceramic phase in-situ in a molten metal phase and holding the mixture at an elevated temperature for a time to form an intergrown ceramic network. The molten metal phase is reactive with a precursor such as gaseous or solid carbon, boron and/or nitrogen in elemental form or as a bearing component to yield products having ceramic characteristics. The network has larger grains than those possible in the present invention.
The present invention overcomes the disadvantages of the prior art noted above. The present invention also permits simplification of procedures and equipment compared to the prior art. For example, the present process obviates the need for multiple furnaces and mixing and control equipment because all of the constituents of the second phase are reacted in a single reaction vessel. The present invention additionally overcomes the need for forming multiple melts of components at very high melting temperatures. Further, high loading composites can be prepared without the necessity of achieving high levels of superheat in holding furnaces.
The invention provides a cleaner particle/metal interface compared with conventional metal-ceramic composites made by techniques using, for example, separate metal and ceramic powders, because the reinforcing particles are formed in-situ, and thus permits formation of a ceramic-ceramic composite having an improved interface.
The preceding features permit a mechanism for producing finer second phase particles in the ceramic-ceramic composite than are possible by classical powder routes. Also, the present process obviates the need for intimate mixing, compacting, and sintering of vety fine ceramic powders associated with conventional techniques and avoids non-uniform green density, differential sintering, and grain growth problems. A cleaner ceramic/ceramic interface is obtained by. the present invention, due to the in-situ formation of the ceramic particles. Using certain embodiments of the invention, grain growth and interparticle reactions often encountered in complex systems are avoided because a single phase is provided for subsequent treatment, that is, the precipitates from the second phase-forming reaction may serve as nuclei for the ceramic matrix forming reaction and become encapsulated.
With these facts in mind, a detailed description of the invention follows, which achieves advantages over known processes and products.